TWO-PATH SUCCESSIVE RELAYING SCHEMES IN THE PRESENCE OF

INTER-RELAY INTERFERENCE

LIAU QIAN YU

UNIVERSITI TEKNOLOGI MALAYSIA

TWO-PATH SUCCESSIVE RELAYING SCHEMES IN THE PRESENCE OF

INTER-RELAY INTERFERENCE

LIAU QIAN YU

A thesis submitted in fulfilment of the

requirements for the award of the degree of

Master of Engineering (Electrical)

Faculty of Electrical Engineering

Universiti Teknologi Malaysia

JANUARY 2016

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To my parents, brother, family and friends.

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ACKNOWLEDGEMENT

First of all, I would like to express my deepest gratitude to Dr. Leow Chee Yen,

my supervisor. I have been extremely lucky to have a supervisor who responded to

my questions and queries so promptly, and who provided encouragement, advice and

support throughout my time as his student. His excellent guidance enables me to finish

my thesis.

Last, but most importantly, I thank my parents and brother. I am very fortunate

and blessed to have such caring family. They were always supporting me and

encouraging me with their best wishes. I would never have been able to finish my

thesis without the support from them.

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ABSTRACT

Relaying is a promising technique to improve wireless network performance.

A conventional relay transmits and receives signals in two orthogonal channels due to

half duplex constraint of wireless network. This results in inefficient use of spectral

resources. Two-Path Successive Relaying (TPSR) has been proposed to recover loss

in spectral efficiency. However, the performance of TPSR is degraded by Inter-Relay

Interference (IRI). This thesis investigates the performance of TPSR affected by IRI

and proposes several schemes to improve relaying reliability, throughput and secrecy.

Simulations revealed that the existing TPSR could perform worse than the conventional

Half Duplex Relaying (HDR) scheme. Opportunistic TPSR schemes are proposed to

improve the capacity performance. Several relay pair selection criteria are developed

to ensure the selection of the best performing relay pair. Adaptive schemes which

dynamically switch between TPSR and conventional HDR are proposed to further

improve the performance. Simulation and analytical results show that the proposed

schemes can achieve up to 45% ergodic capacity improvement and lower outage

probability compared to baseline schemes, while achieving the maximum diversity

and multiplexing tradeoff of the multi-input single-output channel. In addition, this

thesis proposes secrecy TPSR schemes to protect secrecy of wireless transmission

from eavesdropper. The use of two relays in the proposed schemes deliver more robust

secrecy transmission while the use of scheduled jamming signals improves secrecy

rate. Simulation and analytical results reveal that the proposed schemes can achieve

up to 62% ergodic secrecy capacity improvement and quadratically lower intercept

and secrecy outage probabilities if compared to existing schemes. Overall, this

thesis demonstrates that the proposed TPSR schemes are able to deliver performance

improvement in terms of throughput, reliability and secrecy in the presence of IRI.

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ABSTRAK

Penggegantian adalah satu teknik yang menjanjikan peningkatan kepada

prestasi rangkaian wayarles. Geganti konvensional menghantar dan menerima

isyarat dalam dua ortogon saluran kerana kekangan separuh dupleks. Ini

menyebabkan penggunaan sumber spektrum yang tidak cekap. Penggegantian

Dwi-Laluan Berturutan (TPSR) telah dikemukakan untuk memulihkan kehilangan

dalam kecekapan spektrum. Walau bagaimanapun, TPSR mengalami kemerosotan

prestasi disebabkan oleh isyarat gangguan antara geganti (IRI). Tesis ini mengkaji

prestasi TPSR yang terjejas oleh IRI dan mencadangkan beberapa skema untuk

meningkatkan kebolehpercayaan, kelajuan dan kerahsiaan. Kajian simulasi

menunjukkan bahawa skema TPSR yang sedia ada menunjukkan prestasi lebih teruk

daripada skema Penggegantian Separuh Dupleks (HDR) konvensional. Skema-skema

TPSR oportunistik dicadangkan untuk meningkatkan prestasi kapasiti dalam senario

IRI. Beberapa kriteria pemilihan pasangan geganti dibangunkan untuk memastikan

pasangan geganti yang dipilih menyampaikan prestasi yang terbaik. Skema-skema

penyesuaian yang dinamik bertukar antara skema TPSR dan skema HDR konvensional

telah dicadangkan untuk meningkatkan lagi prestasi. Keputusan simulasi dan analisis

menunjukkan bahawa skema-skema yang dicadangkan menyampaikan peningkatan

sehingga 45% dalam kapasiti ergodik dan kebarangkalian gangguan yang lebih

rendah berbanding skema-skema yang sedia ada, manakala mencapai kepelbagaian

dan pemultipleksan yang maksimum bagi saluran berbilang-input tunggal-output.

Di samping itu, tesis ini mencadangkan skema-skema TPSR kerahsiaan untuk

melindungi keselamatan penghantaran wayarles. Skema-skema menggunakan dua

geganti dicadangkan dalam tesis ini untuk memastikan penghantaran kerahsiaan yang

lebih mantap manakala penggunaan isyarat penyesakan berjadual dapat meningkatkan

kadar kerahsiaan. Keputusan simulasi dan analisis menunjukkan bahawa skema-

skema kerahsiaan yang dicadangkan boleh mencapai peningkatan sehingga 62% dalam

kapasiti kerahsiaan ergodik serta kebarangkalian memintas dan kerahsiaan gangguan

yang secara kuadratiknya lebih rendah berbanding dengan skema-skema sedia

ada. Secara keseluruhan, tesis ini menunjukkan bahawa skema-skema TPSR yang

dicadangkan mampu mencapai peningkatan prestasi daripada segi kebolehpercayaan,

kelajuan dan kerahsiaan dalam senario kewujudan isyarat gangguan antara geganti.

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TABLE OF CONTENTS

CHAPTER TITLE PAGE

DECLARATION iiDEDICATION iiiACKNOWLEDGEMENT ivABSTRACT vABSTRAK viTABLE OF CONTENTS viiLIST OF FIGURES xiLIST OF TABLES xivLIST OF ABBREVIATIONS xvLIST OF SYMBOLS xvi

1 INTRODUCTION 11.1 Introduction 11.2 Problem Statements 3

1.2.1 The detrimental effect of inter-relayinterference in TPSR 3

1.2.2 The issues of relay selection in TPSR 41.2.3 TPSR in secrecy wireless communication 5

1.3 Objectives 61.4 Scopes 61.5 Contributions of the Thesis 81.6 Outlines of the Thesis 9

2 LITERATURE REVIEW 102.1 Diversity Techniques to Improve Reliability 102.2 Cooperative Communication 112.3 Successive Relaying 132.4 Opportunistic Relay Selection 152.5 Wireless Physical Layer Security 16

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2.6 Improving Physical Layer Security Using Coopera-tive Nodes 18

2.7 Capacity of Wireless Channels 212.8 Diversity and Multiplexing Tradeoff 222.9 Related Works 23

3 RESEARCH METHODOLOGY 243.1 Research Process 243.2 System Model 26

3.2.1 Channel Model 273.2.2 Discrete-Time Baseband Model 28

3.3 Monte Carlo Simulation 283.4 Derivation of Analytical Results 29

4 PERFORMANCE OF TWO-PATH SUCCESSIVE RE-LAYING IN THE PRESENCE OF INTER-RELAYINTERFERENCE 324.1 Introduction 324.2 System Model 344.3 Two-Path Successive Relaying 34

4.3.1 Transmission Protocol 354.3.2 Instantaneous End-to-End Capacity of

TPSR schemes 364.3.2.1 TPSR scheme 364.3.2.2 TPSR-IC scheme 37

4.4 Half-Duplex Relaying 384.5 Numerical Results 394.6 Chapter Summary 44

5 OPPORTUNISTIC TWO-PATH SUCCESSIVE RELAY-ING SCHEMES IN THE PRESENCE OF INTER-RELAY INTERFERENCE 465.1 Introduction 465.2 System Model 485.3 Protocol Description 48

5.3.1 Initialisation Phase 495.3.2 Relaying Phase 505.3.3 Instantaneous End-to-End Capacity 51

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5.4 Relay Pair Selection Criteria of the Proposed OSRSchemes 525.4.1 Proposed OSR Scheme 525.4.2 Proposed OSR-IC Scheme 535.4.3 Proposed Adaptive OSR Scheme 535.4.4 Proposed Adaptive OSR-IC scheme 55

5.5 Baseline Schemes 555.5.1 Opportunistic Half-Duplex Relaying 565.5.2 Full-Duplex Relaying with Self-

interference 565.5.3 Existing OSR 57

5.6 Analytical Results 575.6.1 Ergodic Capacity Analysis 585.6.2 Outage Probability and the Diversity and

Multiplexing Tradeoff Analysis 625.7 Numerical Results 665.8 Chapter Summary 75

6 TWO-PATH SUCCESSIVE RELAYING SCHEMESFOR SECRECY COMMUNICATION 766.1 Introduction 766.2 System Model 796.3 Proposed Secrecy Two-Path Successive Relaying

Scheme 806.3.1 Transmission Protocol 806.3.2 Secrecy Capacity 826.3.3 Analysis on Intercept Probability 84

6.4 Proposed Secrecy Two-Path Successive RelayingWith Scheduled Jamming Scheme 886.4.1 Transmission Protocol 886.4.2 Secrecy Capacity 896.4.3 Analysis on Secrecy Outage Probability 91

6.5 Baselines Schemes 946.5.1 Secrecy Half-Duplex Relaying Scheme 946.5.2 Secrecy Full-Duplex Relaying Scheme 956.5.3 Secrecy Full-Duplex Jamming Scheme 97

6.6 Numerical Results 986.7 Chapter Summary 107

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7 CONCLUSIONS AND FUTURE WORK 1087.1 Conclusions 1087.2 Future Work 110

REFERENCES 112Appendix A 119

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LIST OF FIGURES

FIGURE NO. TITLE PAGE

2.1 Cooperative communication scenario: a source, S iscommunicating with a destination, D assisted by a relay, R. 12

2.2 TPSR scenario: a source, S is communicating with adestination, D assisted by two relays, Ra and R

b. 14

2.3 Wiretap channel model. 172.4 A source transmitter, S is communicating with a destination

receiver, D assisted by a half-duplex relay, R. Aneavesdropper, E overheard the transmitted signal from R. 19

2.5 A source transmitter, S is communicating with a destinationreceiver, D assisted by a selected half-duplex relay, R anda jammer, J . An eavesdropper, E overheard the transmittedsignal from R. 20

3.1 The research process. 263.2 The process of analytical results derivation. 314.1 TPSR scenario: a source, S is communicating with a

destination, D assisted by two relays, Ra and Rb. 35

4.2 HDR scenario: a source, S is communicating with adestination, D assisted by a relay, R. 38

4.3 Ergodic capacity versus SNR of TPSR, TPSR-IC and HDRschemes when V = 0 dB. 39

4.4 Outage probability versus SNR of TPSR, TPSR-IC and HDRschemes when V = 0 dB and target rate, R = 1 bits/s/Hz. 40

4.5 Ergodic capacity versus SNR of TPSR, TPSR-IC and HDRschemes when V = −10 dB. 41

4.6 Outage probability versus SNR of TPSR, TPSR-IC and HDRschemes when V = −10 dB and target rate, R = 1 bits/s/Hz. 42

4.7 Ergodic capacity versus SNR for TPSR, TPSR-IC and HDRschemes when V = 10 dB. 43

4.8 Outage probability versus SNR for TPSR, TPSR-IC and HDRschemes when V = 10 dB and target rate, R = 1 bits/s/Hz. 44

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5.1 The system model of successive relaying. 495.2 Mean of inter-relay interference channel of the selected best

relay pair,∣∣∣hRa,Rb ∣∣∣2 versus the number of relays, N for the

proposed OSR scheme. 635.3 Ergodic capacity versus the number of potential relays, N of

the proposed OSR scheme in comparison with the capacitybounds when SNR = 60 dB. 67

5.4 Ergodic capacity versus SNR of various schemes when thenumber of potential relays, N = 30. 68

5.5 Outage probability, Pout versus SNR of various schemes whenthe number of potential relays, N = 30 and the target rate,R = 3 bits/s/Hz. 69

5.6 Ergodic capacity versus SNR of various schemes when thenumber of potential relays, N = 10. 70

5.7 Outage probability, Pout versus SNR of various schemes whenthe number of potential relays, N = 10 and the target rate,R = 1 bits/s/Hz. 71

5.8 Ergodic capacity versus the number of potential relays, N ofvarious schemes when SNR = 30dB. 72

5.9 Outage probability, Pout versus the target rate, R for variousschemes when the number of potential relays, N = 30 andSNR = 30 dB. 73

5.10 Ergodic capacity versus the variance of inter-relay channel,V when SNR = 30 dB and the number of potential relays,N = 30. 74

6.1 The secrecy two-path successive relaying (TPSR) networkwith an eavesdropper. 80

6.2 Ergodic secrecy capacity versus SNR where γsr = γrd, γse =

γre = 10 dB and γRR = γrr = 0 dB. 996.3 Ergodic secrecy capacity versus SNR where γsr = γrd, γse =

γre = 40 dB and γRR = γrr = 0 dB. 1006.4 Intercept probability versus SNR where γsr = γrd, γse =

γre = 10 dB and γRR = γrr = 0 dB. 1016.5 Intercept probability versus γRR or γrr where γsr = γrd =

40 dB and γse = γre = 10 dB. 1026.6 Secrecy outage probability versus SNR where γsr = γrd,

γse = γre = 10 dB and γRR = γrr = 0 dB. 103

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6.7 Secrecy outage probability SNR where target secrecy rate,r = 1 bits/s/Hz, γsr = γrd, γse = γre = 40 dB andγRR = γrr = 0 dB. 104

6.8 Secrecy outage probability versus target secrecy rate, r whereγse = γre = 10 dB, γsr = γrd = 40 dB and γRR = γrr =

0 dB. 1056.9 Secrecy Outage probability versus γRR and γrr where target

secrecy rate, r = 2 bits/s/Hz, γse = γre = 10 dB and γsr =

γrd = 40 dB. 106

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LIST OF TABLES

TABLE NO. TITLE PAGE

2.1 Existing Techniques of TPSR 23

2.2 Existing Secrecy Relaying Schemes 23

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LIST OF ABBREVIATIONS

5G - Fifth Generation

ADC - Analog-to-Digital Converter

AF - Amplify-and-Forward

CDF - Cumulative Density Function

CSI - Channel State Information

IoT - Internet of Things

HDR - Half-Duplex Relaying

RF - Radio Frequency

TPSR - Two-Path Successive Relaying

IC - Successive Interference Cancellation

DF - Decode-and-Forward

DMT - Diversity and Multiplexing Tradeoff

FDR - Full-Duplex Relaying

FDJ - Full-Duplex Jamming

HDC - Half-Duplex Constraint

i.i.d. - Independent and Identically Distributed

OHR - Opportunistic Half-Duplex Relaying

OSR - Opportunistic Two-Path Successive Relaying

PDF - Probability Density Function

SISO - Single-Input and Single-Output

SR - Successive Relaying

SNR - Signal-to-Noise Ratio

SINR - Signal-to-Interference-Plus-Noise Ratio

TDD - Time-Division-Duplex

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LIST OF SYMBOLS

u - Lower case letter denote scalars

u - Boldface lower case letters denote vectors

U - Boldface upper case letters denote matrices

Pr (A) - Probability of event A(nk

)- Binomial coefficient indexed by n and k

det(.) - Determinant

exp(.) - Exponent

Ei (·) - Exponential integral function

log2

- Logarithm with base 2

log - Logarithm with base 10

lnx - Natural logarithm

[.]T - Transpose operation

[.]H - Hermitian transpose operation

|.| - Absolute value

[.]+ - max (0, x)

∈ - is an element of

x̄ - Statistical expected value

→ - Approaches.= - Exponential equality

≈ - Approximately equal

, - Equal by definition

{ , } - the set of

∼ - is distributed as

CN (µ, σ2) - Complex Gaussian distribution with mean µ and varianceσ2

CHAPTER 1

INTRODUCTION

This chapter begins with the introduction of this thesis in Section 1.1. Problemstatements are presented in Section 1.2. Section 1.3 and 1.4 describe the objectives andscopes of this thesis respectively. Finally, the contributions and outlines of this thesisare highlighted in Section 1.5.

1.1 Introduction

The fifth generation (5G) wireless network will serve as a key enabler inmeeting the ever increasing demand for data rates in future wireless applications. 5G isenvisioned to deliver not only ultra-high data rate, but also ultra-wide radio coverage,ultra-large number of devices, and ultra-low latency [1]. 5G supports device-to-deviceand machine-to-machine communications, which contributes to the development ofInternet of Things (IoT) [2]. In IoT, a large number of devices and machines withsensors and/or actuators are connected to the internet to form a highly dense network.In the dense network, a number of idle devices or machines with no message to transmitor receive can actively assist the network by assuming the role of relays. Relaysoffer additional paths for message transmission between the source and destination,subsequently improve the robustness of the transmission [3].

A transmission assisted by a relay, or more commonly known as cooperativecommunication, is introduced to improve the reliability of wireless transmission.In cooperative communication, relay assists the transmission by offering alternativeindependent transmission path between the source and the destination. Theindependent path delivers spatial diversity to help the communication system toovercome shadowing, deep fade and multipath. In cooperative communication, therequirement for a conventional relay to transmit and receive signals simultaneously in

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the same channel is traditionally assumed to be impractical. It is established that thepower of the intended received signal of the relay is much lower than the power ofthe transmitted signal of the relay [4]. When operating simultaneously in the samechannel, the self-transmitted signal saturates the receiver amplifier and analog-to-digital converter (ADC) and the relay is unable to isolate the intended received signalfrom the self-transmitted signal. In order to prevent this issue, the relay receives andtransmits signals in two orthogonal frequency channels or time slots. The requirementto isolate the transmit and receive operations is generally known as the half-duplexconstraint.

A source has to stop transmission of new message when the relay istransmitting message to the destination, due to the half-duplex constraint. Otherwise,the message transmitted by the source during the relay transmission phase will not becorrectly received by the relay. This transmission scheme is also called half-duplexrelaying (HDR). The HDR transmission requires double amount of channel resourcescompared to a direct transmission from source to destination without relay. As aresult, the spectral efficiency of HDR is at most half of the spectral efficiency of directtransmission.

Full-duplex relay has been proposed to improve the bandwidth efficiency. Atypical full-duplex relay is equipped with two antennas and two radio frequency (RF)chains used to transmit and receive signals respectively. This allows the full-duplexrelay to transmit and receives signals simultaneously in the same channel. However,this comes at a cost of self-interference at the relay. The transmitted signal at thetransmit antenna interferes the received signal at the receive antenna. Recent literatureshows that the self-interference can be minimised and the residual interference maybe regarded as additive noise [5, 6]. Advanced signal isolation techniques in theanalog, digital, and propagation domains are required by the full-duplex relay tosuppress the self-interference. Such techniques require sophisticated hardware and/oradvanced signal processing which significantly increases the cost and complexity ofrelay nodes [7]. This contradicts the original motivation of using relays to provide alow complexity and inexpensive solution to improve the wireless transmission [8].

Successive relaying protocols are introduced to improve the spectral efficiencyusing only conventional half-duplex relays [9–11]. In successive relaying protocols,multiple half duplex relays are scheduled to assist the source transmissioncontinuously. One of the popular successive relaying protocols is known as two-pathsuccessive relaying (TPSR) [11]. In TPSR, two conventional relays, Ra and Rb are

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scheduled to assist the transmission from source S to destination D alternately. Whenone of the relays is transmitting message to the destination, the other relay goes intoreceiving mode to receive the message transmitted from the source. TPSR allows thesource and destination to transmit and receive new messages continuously. As a result,TPSR can achieve the same spectral efficiency as full-duplex relaying. However, whenoperating in co-channel, the transmitted signal from the transmitting relay interferethe received signal of the receiving relay. This interference is known as inter-relayinterference and it causes the performance bottleneck in TPSR.

On the other hand, owing to the broadcast nature of wireless transmission, thewireless security remains one of the main concerns in wireless communication. Inwireless communication, a transmitted signal from the source can be readily overheardby an unauthorised node. The transmitted signal is not secured when the unauthorisednode intercepts the signal. The unauthorised node with the purpose to intercept thetransmission is known as eavesdropper. The presence of eavesdropper poses a seriouschallenge to the security of wireless transmission. Traditionally, information securityis addressed at upper layers of the network protocol stack such as application layer,transport layer and networking layer, based on cryptography methods. The general ideaof cryptography is to protect the message so that unauthorised nodes without a securitykey can gain no information of the encrypted message. However, an eavesdropper withextremely high computational capability is still able to intercept the encrypted messagethrough an exhaustive key search. Recently, physical layer security is identified as apromising technique that secure the wireless transmission by exploiting the physicalcharacteristics of the wireless channel. Relaying approach has also been proposed toenhance the secrecy of wireless transmission [12–14].

1.2 Problem Statements

This section presents the problem statements of this thesis. The problemstatements are described in the following subsections.

1.2.1 The detrimental effect of inter-relay interference in TPSR

In TPSR scheme, two relays are scheduled to transmit and receive alternatelyto imitate the full-duplex relay to deliver continuous source transmission to the

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destination. As a result, TPSR can deliver the same spectral efficiency as the full-duplex relaying. This motivates the use of TPSR to address the cost and complexityof full-duplex relay. However, when operating in co-channel, the received signal ofthe receiving relay is interfered by the transmitted signal from the transmitting relay.This inter-relay interference degrades the performance of TPSR. In the early literature,the inter-relay interference is mitigated by operating the two relays in two orthogonalfrequency channels [15]. However, the use of two orthogonal channels decreases thespectral efficiency of TPSR to half of the spectral efficiency of full-duplex relaying.This diverges from the original purpose of TPSR to achieve the spectral efficiencyof full-duplex relaying. In [11], successive interference cancellation (IC) decodingstrategy is proposed to minimise the inter-relay interference. In the IC decodingstrategy, the relays decode the inter-relay interference and subtract it from the receivedsignal, before proceed to decode the message transmitted from the source. However,the IC decoding strategy is only effective when the power of inter-relay interference ismuch stronger than the power of the intended signal from the source. Existing literaturedoes not compare the ergodic capacity and outage probability of TPSR against HDRin various channel and interference conditions [11]. It is therefore a need to comparethe performance of TPSR affected by inter-relay interference with the HDR in terms ofergodic capacity and outage probability in various channel and interference conditions.The performance investigation of TPSR is presented in Chapter 4.

1.2.2 The issues of relay selection in TPSR

In TPSR, two relays assist the transmission alternately to imitate the operationof a full-duplex relay. This enables TPSR to achieve the spectral efficiency of full-duplex relaying. However, when operating in co-channel, the alternate transmit andreceive operations of the two relays generate interference to each other. The inter-relayinterference is the main contributing factor to the performance bottleneck of TPSR interms of ergodic capacity and outage probability. Existing literature employs relaypair selection techniques to improve the ergodic capacity, outage probability and thediversity-and-multiplexing tradeoff of TPSR [16, 17]. In [16] and [17], two relays areselected from N relays in initialisation phase using relay pair selection criteria. Therelay pair selection criteria affect the performance of TPSR.

In [16], two relays are selected individually with different criteria. First, therelay with the highest max-min capacity of source-to-relay channel and relay-to-destination channel is selected as the first relay, without considering the inter-relay

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interference. The second relay is selected from a decoding set of relays, D formedby the remaining relays which can decode the inter-relay interference and performIC decoding of the source message. The qualified relays in D with the highest end-to-end capacity is then selected as the second relay. The individual selection of therelays reduces the pool of available relay pairs from

(N2

)= N (N − 1) /2 to N − 1.

Consequently, only 2/N of the available relay pairs are considered in the selectionprocess. As a result, the best relay pair which achieves the highest capacity might notbe considered in the selection process.

In [17], the inter-relay interference is utilised for superposition coding toprovide additional diversity. A relay pair is qualified to perform superposition codingonly when the source message and the inter-relay interference can be decoded by bothrelays. From the qualified relays, the relays with the largest and the second largestinstantaneous capacity of the relay-to-destination channels are selected. Due to thestrict requirement, the initialisation phase in [17] requires a total 1+N2+2 log2N bitsof overhead to acquire channel state information (CSI) of the relays and select the relaypair. In addition, the instantaneous end-to-end capacity is not considered in the relayselection. Therefore, the selected relay pair might not be the relay pair that achievesthe highest capacity. On the other hand, the strict requirement of superposition codingmay result in no relay pair being selected. As discussed in [17], when there is noqualified relay pair, the transmission mode falls back to the conventional HDR and anew relay needs to be selected. This further increases the overhead of the transmission.The use of capacity-wise suboptimal selection criteria in [16] and [17] motivates theproposal of new opportunistic TPSR schemes in Chapter 5.

In addition, based on the results in Chapter 4, TPSR does not alwaysoutperform HDR. Under certain channel conditions, HDR achieves higher ergodiccapacity and lower outage probability than TPSR. Adaptive switching between TPSRand HDR modes has not been considered in the literature. This motivates the proposalof new adaptive TPSR schemes in Chapter 5.

1.2.3 TPSR in secrecy wireless communication

Relay provides substantial benefits not only in terms of reliability and spectralefficiency, but also beneficial in enhancing the secrecy of wireless transmission viaphysical layer security [12–14]. Physical layer security exploits the characteristics of

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the wireless channel such as channel fading and interference to improve transmissionsecurity. The existing literature on physical layer security mainly focuses on HDR[18–20]. In secrecy HDR, the relay cannot transmit and receive signal simultaneouslyin the same frequency channel due to the half-duplex constraint. This limits theperformance of secrecy HDR. Recently, full-duplex relaying is proposed to improvetransmission security [21]. The secrecy full-duplex relaying achieves higher secrecycapacity and lower secrecy outage probability than the secrecy HDR. This is becausethe full-duplex relay can transmit and receive signal simultaneously in the samefrequency channel. However, this comes at a cost of self-interference because thereception of the full-duplex relay is interfered by its own transmission. Advancedsignal isolation techniques in the analog, digital, and propagation domains are requiredto suppress the self-interference and this significantly increases the cost and complexityof full-duplex relay [7]. Alternatively, TPSR is proposed to imitate the full-duplexrelaying by scheduling the operation of two conventional half duplex relays [11].However, the use of TPSR for secrecy communication has not been considered in theliterature and its secrecy performance remains unknown. This motivates the proposalof secrecy TPSR schemes in Chapter 6 to provide performance improvement in termsof secrecy ergodic capacity, secrecy outage probability and intercept probability.

1.3 Objectives

The objectives of this thesis are laid out as follows,

1. to investigate the effect of inter-relay interference to the ergodic capacity andoutage probability of TPSR.

2. to propose opportunistic TPSR schemes with relay pair selection to improve theergodic capacity and outage probability.

3. to propose secrecy TPSR schemes to improve the ergodic secrecy capacity,intercept probability and secrecy outage probability.

1.4 Scopes

This thesis considers the single-input and single-output (SISO) communicationscenarios with a source and a destination assisted by two half-duplex relays. This thesis

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only considers a single-hop relaying scenario because a multi-hop relaying scenarioprovides a very limited gain in the achievable data rate compared to the single-hoprelaying scenario [22]. The half-duplex relays cannot transmit and receive signalsimultaneously in the same frequency channel due to half duplex constraint. Therelays apply decode-and-forward (DF) strategy to assist the transmission. By using DFstrategy, the relays decode the messages from the source, re-encode and then transmitsto destination. The DF strategy avoids amplified noise in the relayed signal. Thetransmit power of the source and relays are fixed to unity. All the nodes are equippedwith single antenna. The receivers of the relays and destination are corrupted bycomplex circularly symmetric additive white Gaussian noise in real and imaginarycomponents with distribution CN (0, σ2). All channels are reciprocal and followquasi-static, frequency flat Rayleigh fading distribution. The channels are independentand identically distributed (i.i.d.), unless stated otherwise. In the secrecy transmissionscenario, an eavesdropper equipped with a single antenna without jamming capabilityis considered. Without loss of generality, it is assumed that the direct channel betweensource and destination does not exist due to severe shadowing and/or extreme pathloss, as in the existing literature [16, 17, 21].

The performance of the proposed schemes are simulated in MATLAB softwareusing Monte Carlo technique. Each of the Monte Carlo simulations take 100,000 trialsto ensure the accuracy of the simulation results. The average signal-to-noise ratio(SNR) is defined as 1/σ2 in the simulations. The performance metrics measured bythe simulations in Chapter 4 and 5 are ergodic capacity and outage probability. InChapter 5, tradeoff between target rate and outage probability is evaluated to validatethe diversity-and-multiplexing tradeoff. Meanwhile, the ergodic secrecy capacity,intercept probability and secrecy outage probability are considered in Chapter 6. Theperformance of the proposed schemes also are evaluated analytically using informationtheory and statistical tools. During the derivation of the analytical results, the tableof integrals in [23] serves as a reference in solving complex integration problemsand the order statistics in [24] is used to quantify the distribution of the maximumand minimum values. In Chapter 5, the closed form equations for ergodic capacity,outage probability and diversity-and-multiplexing tradeoff of the proposed schemesare derived. Whereas, the analytical equations for intercept probability and secrecyoutage probability of proposed schemes in Chapter 6 are derived.

In this thesis, multiple-input and multiple-output (MIMO) communicationand power allocation are not considered. This thesis evaluates the performance ofthe proposed schemes in worst case scenario using Rayleigh fading model, without

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considering the effect of path loss.The transmission for channels with fast fading isnot included in this thesis. The various channel conditions in Chapter 4 and 5 referto the various levels of inter-relay interference. For the proposed secrecy transmissionschemes in Chapter 6, the design of wiretap coding is not considered in this thesis.

1.5 Contributions of the Thesis

The original contributions of this thesis can be summarised as follow,

• The ergodic capacity and outage probability of the existing TPSR schemesaffected by inter-relay interference are investigated in various channel andinterference conditions characterised by different interference levels andcompared to HDR in Chapter 4. The results reveal that TPSR does not alwaysperform better than conventional HDR.

• New relay pair selection criteria based on the instantaneous end-to-end capacityare proposed in Chapter 5.

• New opportunistic TPSR schemes with relay pair selection are proposed inChapter 5. The proposed opportunistic TPSR schemes are compared to existingopportunistic HDR, opportunistic TPSR schemes and full-duplex relayingschemes numerically in terms of ergodic capacity and outage probability.Information-theoretic analytical results on ergodic capacity, outage probabilityand diversity-and-multiplexing tradeoff of the proposed opportunistic TPSRschemes are derived.

• New adaptive opportunistic TPSR schemes are proposed in Chapter 5 tofurther improve the ergodic capacity and outage probability in various channelconditions characterised by different interference levels.

• New secrecy TPSR schemes are proposed in Chapter 6. The proposed secrecyTPSR schemes are compared to existing secrecy HDR and secrecy full-duplexrelaying schemes numerically in terms of ergodic secrecy capacity, interceptprobability and secrecy outage probability. Information-theoretic analyticalresults on intercept probability and secrecy outage probability of the proposedsecrecy TPSR schemes are derived.

9

1.6 Outlines of the Thesis

The rest of this thesis is organised as follows. Chapter 2 gives an overview onthe background and covers the literature review of the research topics in this thesis.Chapter 3 describes the research methodology of this research. Chapter 4 investigatesthe performance of TPSR in the presence of inter-relay interference. Chapter 5 presentsthe proposed opportunistic TPSR schemes. Chapter 6 describes the proposed secrecyTPSR schemes. Chapter 7 concludes the thesis and recommends several future work.

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